Sepsis is an extremely prevalent and often fatal disorder that can arise from dysregulation in a host's immune response to bacterial infection. Being the first line of defense against infection, inflammation is a necessary part of a healthy immune response, but too much inflammation can trigger a cascade of pathophysiological processes for which the affected patient cannot adequately compensate, resulting in a diagnosis of sepsis. There are an estimated 750,000 cases of sepsis per year, with mortality rates ranging from 20-50%. Importantly, recent studies indicate that the incidence of sepsis is rising and is projected to continue as the population ages. Understanding the mechanisms and pathways regulating immunity against bacterial infection, then, is imperative. Based on our preliminary investigations into such mechanisms as they pertain to sepsis, we hypothesize that chymase, a mast cell-specific protease, limits the magnitude of the inflammatory response against bacteria and reduces the risks of severe sepsis. Specifically, our data indicate that chymase is able to limit the net negative effects of pro-inflammatory mediators like tumor necrosis factor (TNF)?-through proteolytic cleavage limiting TNF? levels and receptor activation in vivo-in the cecal ligation and puncture (CLP) model of sepsis. Moreover, our preliminary studies indicate that we can use proteomics approaches to identify novel chymase-regulated mediators as part of our effort to elucidate the roles of chymase in altering the immune response and regulating inflammation during sepsis. Accordingly, we propose a research plan aimed at dissecting the mechanisms by which chymase protects its host in sepsis and at defining the factors that can limit this protective effect. In Aim 1, we will define the factors that enable chymase to regulate inflammation and promote survival in sepsis. Specifically, we will determine the mechanisms that induce mast cell activation and chymase release in sepsis. In Aim 2, we will assess the extent to which chymase-mediated down-regulation of TNF? in sepsis can be attributed to direct proteolysis. In Aim 3, we will identify physiologically relevant proteolytic substrates for chymase to establish the overall role of this protease in sepsis. Together, these studies have the potential to provide protein signatures that define the septic inflammatory response and can be used for more effective diagnosis and treatment of this complex disorder. We are confident that these studies will lay the groundwork for future projects aimed at understanding the cascade of events that initiate a dysregulated inflammatory response and lead to multiple organ dysfunction and death in severe sepsis.